One technique that I see a lot on this mailing list is people putting
auto-incremented integer primary keys on their tables.
Maybe I'm just "old school" but I've always thought that you should choose a
primary key based on data that is actually in the table whenever possible,
rather than generating a new value out of thin air.
The only exception that comes to mind is things like ID numbers; for
example, it is better to use an internally-generated integer for an employee
number than it is to use an employee's name. Even the combination of first
name and last name is not necessarily unique - I could cite a real life
example -and, of course, people can change their names. That makes names
less desireable than a generated value when you are trying to uniquely
indentify such entities. In such a case, a nice, reasonable short integer is
easier.
I just found this rather good definition of primary keys at
http://www.utexas.edu/its/windows/database/datamodeling/dm/keys.html. The
relevant bit says that a primary key must have:
- a non-null value for each instance of the entity
- a value that is unique for each instance of an entity
- a value that must not change or become null during the life of the each
instance of the entity
That article makes the same basic remarks about name vs. ID but makes the
point that it is more commonly the case that table designers will use
something like a social security number - an _externally_ generated number -
to distinguish between employees rather than an internally-generated number.
But the trend in this mailing list is toward using generated values as
primary keys in virtually EVERY table, even when good primary keys can be
found in the (non-generated) data already existing in the table.
Now, I haven't done anything remotely resembling a quantified analysis so
maybe I'm wildly exaggerating this trend. But I do seem to recall a lot of
table descriptions with auto-generated keys and I don't think they were all
a name vs. ID scenario....
Has anyone else noticed a similar trend?
If this trend is real, it doesn't seem like a very good trend to me. For
example, if you were keeping track of parts in a warehouse, why would anyone
make a table that looked like this:
ID (autogenerated PK) PART_NO PART_DESCRIPTION
1 A01 Widget
2 B03 Grapple Grommet
3 A02 Snow Shovel
4 D11 Whisk
5 C04 Duct Tape
when this table is simpler:
PART_NO (PK) PART_DESCRIPTION
A01 Widget
B03 Grapple Grommet
A02 Snow Shovel
D11 Whisk
C04 Duct Tape
Would anyone care to convince me that the first version of the table is
"better" than the second version in some way?
I just want to be sure that no one has come along with some new and
compelling reason to autogenerate keys when perfectly good keys can be found
within the data already. I don't mind being "old school" but I don't want to
be "out to lunch" :-)
Rhino
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